Float out ExprSelect to larger scope

[roberth]

Is your feature request related to a problem? Please describe.

Current evaluator performance slightly favors

let inherit (lib) foo;
in f foo foo foo;

instead of

f lib.foo lib.foo lib.foo

Actually writing out the let binding can be perceived as a chore, and having to think about it, or ask for it in lib maintenance is not ideal.

Describe the solution you'd like

Perhaps perform this transformation behind the scenes.
In some cases, the transformation may be a clear win. What would be the algorithm that determines this? Is it fast enough to perform eagerly?

Describe alternatives you've considered

Keep operational semantics as is. Memory use will be slightly more predictable. File loading remains fast. Code remains simple.

Additional context

• Raised in check-meta: Improve performance nixpkgs#267309 (comment)
• The solution and problem have similarities to Naive thunk closures cause space leaks #8285
  • both are forms of partial evaluation

Priorities

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[nixos-discourse]

This issue has been mentioned on NixOS Discourse. There might be relevant details there:

https://discourse.nixos.org/t/removing-most-uses-of-top-level-with/41233/3